The invention concerns a method for establishing a digital communication link, in particular a digital voice link, between at least two subscriber terminals. The invention further concerns a subscriber terminal and a switching center for establishing the digital communication link.
For digital telephone links, according to the ISDN standard, communication links are configured between two subscriber terminals at a standard transmission rate of 64 kbit/s per B channel. This is described, for example, in the article xe2x80x9cDigitales Vermittlungssystem EWSDxe2x80x9d [EWSD digital switching system] by K. Langner (Telekom Unterrichtsblxc3xa4tter, Vol. 46, 2, 1993). An originating subscriber negotiates the call request via a control channel (D channel) with a switching center, which then sets up the communication link.
According to ITU-T Recommendation I.460 (Facsimile III.8-Rec. I.460, 1993), it is possible in ISDN to use other transmission rates lower than 64 kbit/s, for example 8, 16, or 32 kbit/s, and then to adapt them to the transmission rate of a B channel. There furthermore exists the possibility of multiplexing several data signals with a lower transmission rate into one 64 kbit/s B channel.
It is the object of the invention to provide a method with which it is possible, in a digital telecommunications network, to establish between at least two subscriber terminals a communication link which is better adapted to the transmission bandwidth used by one subscriber. Further objects of the invention are to provide a subscriber terminal and a switching center which are suitable for carrying out the method.
The object is achieved by a method for establishing a digital communication link between an originating subscriber terminal and at least one called subscriber terminal, having the steps of requesting from a switching center a link from the originating subscriber terminal to the called subscriber terminal; and establishing the link at a standard transmission rate, further having the steps of selecting a new, lower value for the transmission rate; agreeing upon the new, lower value with the switching center; and switching the transmission rate of the link over to the new, lower value.
The object is also achieved by a subscriber terminal for establishing a digital communication link to at least one called subscriber terminal, having the following devices: means for requesting from a switching center a provisional link to the called subscriber at a standard transmission rate; means for selecting a new, lower value for the transmission rate; and means for agreeing with the switching center upon the new, lower value.
The object is still further achieved by a switching center for establishing a digital communication link between at least two subscriber terminals, having the following devices: means for establishing, at a standard transmission rate, a provisional link to a called subscriber terminal requested by an originating subscriber terminal; means for agreeing with the originating subscriber terminal upon a new, lower value for the transmission rate; and means for switching the transmission rate of the provisional link over to the new, lower value.
One advantage of the invention is that the available transmission capacity of a telecommunications network can be used more effectively, and that only as much transmission capacity as is actually necessary needs to be made available for a communication link. A further advantage is that if a call metering system depends on the maximum payload that can be transmitted via the communication link that is established, there is a cost economy in terms of call charges for the subscriber.